The invention relates to a process for manufacturing a fine-grained recrystallized sheet that is suitable for superplastic forming made of heat-treatable i.e. age-hardenable aluminum alloy.
The aluminum alloys that qualify as age-hardenable are those with which an increase in strength can be achieved by heat treatment, in contrast to those with which this can be achieved only by cold forming. Alloys of the age-hardenable type include in particular the AlMgSi, AlCuMg, AlCuMgSi, AlZnMg, AlZnMgCu and Li-containing varieties. In these alloys, there is a tendency to form coarse grain if the solution treatment that is necessary for the age-hardening process is associated with recrystallization. For very many applications, especially for superplastic forming, a fine-grained structure is desired or a basic prerequisite. For that reason, sheet materials that should be deformed superplastically are required to have a grain-size of less than 25 .mu.m, preferably less than 10 .mu.m. Furthermore, the grains should be almost equiaxed. In addition, no significant coarsening of the grains should occur during the superplastic deformation which is performed at about 500.degree. C.
A number of process proposals are known for the manufacture of superplastically formable sheet of age-hardenable aluminum alloys. The following process, for example, is known from the U.S. Pat. No. 4,528,042:
Solution treatment of a cast ingot above 460.degree. C., cooling at 11.degree. C./h to 56.degree. C./h to 315.degree.-370.degree. C., hot rolling with a starting temperature between 315.degree. and 370.degree. C. and a finishing temperature below 315.degree. C., cooling to 93.degree.-232.degree. C., hot rolling at low temperatures to final thickness and finally rapidly recrystallize.
This and similar processes have, however, with respect to the resultant grain-size, in particular during superplastic forming, proved to be very sensitive to small variations in the alloy composition and to other process parameters.